In manufacturing operations, accurate positioning of carriers or slides with mechanical processing units to predetermined locations along a fixed linear path is frequently accomplished with rack and pinion gearing. If a single pinion drive is employed, backlash between gear teeth may involve unacceptable error in positioning, particularly where accuracy upon reversal of drive is required.
State of the art anti-backlash operation has been provided with a pair of pinion gears mounted on parallel shafts engaging a common rack with an adjustable rigid dual worm gear drive coupling for the pinion gears adapted to produce preload engagement of opposing gear teeth faces with the rack teeth. The rigid coupling has involved need for repeated readjustment to compensate for wear, as well as variation in preload tooth pressure and possible backlash due to inherent tolerance variations in pitch spacing, rack mounting high and low points or other causes of inaccuracies; also, the worm gearing involves low efficiency particularly objectionable in meeting high load acceleration requirements.
A backlash eliminator is disclosed in U.S. Pat. No. 3,151,494, wherein a pair of pinion gears mounted on parallel shafts engage a common bull gear for a rotary table of a machine tool. A pair of pulleys on the respective gear shafts are coupled by a positive drive belt adapted to produce engagement of opposing gear teeth faces with the bull gear teeth through application of tensioning to one side of the belt. Tensioning of the belt causes it to apply preload torque in opposite directions to each of the pulleys and associated pinion gears in order to eliminate backlash. Once such belt tensioning is applied by rigid bell crank leverage, the tensioner is locked in fixed position with no provision for accommodating tolerance variations in the relative operating dimensions.
A further dual pinion anti-backlash carriage drive for a machine tool is disclosed in U.S. Pat. No. 4,714,388. A carriage or slide of a machine tool is moved along a supporting guideway or way by a pair of driven pinions engaging a rack, and each pinion is mounted on the output shaft of a corresponding planetary gearbox. The input shafts of the gearboxes are driven by a reversible electric motor through an endless belt and pulley system. A pair of belt tension wheels engage the endless belt and are supported by a set of spring biased slide members which cooperate to establish a predetermined tension in the belt and a predetermined preload torque on the input and output shafts of the gearboxes to produce a corresponding anti-backlash force of the pinions against the rack.